There's no bracket system here, unlike income tax. Below the $24,128 annual threshold you repay nothing at all, and above it, every extra dollar you earn has 12% deducted, with no cap and no second rate further up the income scale. It's the simplest deduction on your payslip precisely because it's a single flat rate applied to one number.
Your employer handles this automatically once you tell them (or IRD tells them) you have a student loan, applying an "SL" tax code alongside your normal PAYE code so the repayment comes out every pay run without you having to do anything manually. It sits stacked on top of income tax, the ACC Earner's Levy, and whatever KiwiSaver rate you've chosen, all deducted from the same paycheque.
Because New Zealand student loans are interest-free while you're living in the country, the calculus around extra voluntary repayments is different than in most other countries: there's no interest cost racking up in the background, so paying it off faster is a lifestyle and cashflow choice rather than something that saves you meaningful interest, unless you're planning time overseas, where interest does start to apply.